Abstract : Terrorism is a continuing and disturbing threat. Some consider military retaliation an appropriate response with deterrent value against terrorists and supporters. Terrorism itself as a tactic uses violence to communicate with and influence audiences for political purpose. It is a form of psychological warfare that goes beyond acceptable norms for the use of force in international relations. The choice of the terror tactic involves both conscious and unconscious motivations; it can satisfy both the strategic and psychological needs of its perpetrators. Furthermore, the psychological motivations of terrorists, their values, and their desire to flaunt conventional logic place them on a different level of rationality from nonterrorists. Like terrorism, retaliation is a form of communication through violence. It can affect multiple audiences for many purposes: bolstering public opinion, destroying/disrupting terrorist infrastructure, and potentially deterring the choice of the terrorist tactic. Symmetry, proportionality, and discrimination in the targeting of retaliation all vary its effects on audiences. To deter terrorists and their supporters, retaliation must meet the requirements of deterrence theory: credibility, shared interest, and rationality. Examples of retaliation for terrorism indicate there are significant problems with its effectiveness as a deterrent. Its viability is diminished by the transience and fragility of credibility, the moral and legal 'baggage' of retaliation itself, and the differences in values and interests between terrorists/ supporters and retaliating states. Retaliation also presents substantial risks beyond its failure to deter. Force protection, dangers of escalatory violence, and risks of condemnation by the world community accompany the use of retaliation.